In healthy kidneys, which statement best describes the glomerulus's handling of large proteins?

• A. Large proteins are freely filtered into urine.
• B. Large proteins are normally prevented from passing into filtrate.
• C. Protein reabsorption is complete in the proximal tubule.
• D. Proteins are secreted into urine by the proximal tubule.

Answer: (B)

Large proteins are normally blocked by the glomerular filtration barrier. This barrier acts as a size- and charge-selective sieve, built from a fenestrated endothelium, a negatively charged basement membrane, and podocyte slit diaphragms. Together, they prevent large (and usually negatively charged) proteins from passing into the filtrate, so they stay in the blood. In healthy kidneys, urine contains negligible protein because these large molecules are not filtered. If small amounts of protein do appear in filtrate, the proximal tubule can reabsorb them, but the essential idea is that large proteins are normally kept out of the filtrate.